Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Supercapacitors store energy by means of separation of charge rather than by the electro-chemical process inherent in a battery.
Supercapacitors are also designated by terms such as ultra capacitors, electrochemical double layer capacitors (EDLC) and electrochemical capacitors, amongst others, all of which are included within the term “supercapacitor” as used within this specification.
Supercapacitors generally enable fast (high power) delivery of energy with the amount of energy delivered being very high compared to ordinary capacitors, but low compared to batteries. Low resistance, high energy density, small supercapacitors are ideally suited for high power applications such:                Wireless communications with limited power supplies such as: Mobile/cellular telephones; PC card; CF card; mini PCI; express card; USB modems; PDA's; automatic meter reading; toll tags; GPS, GPRS and RF tracking.        Energy back-up (UPS) in portable, or space constrained devices.        Voltage regulation for CPU's; automotive vehicles; portable audio and other devices with high surge loads.        High energy, high power electrical loads, such as: Actuators for door locks; DSC's and LED flash for cameras.        Solid state memory storage devices (eg. solid state hard drives).        
Supercapacitors generally include two opposed electrodes electrically isolated by an intermediate electronically insulating separator which is porous and permeated by the electrolyte. Two current collecting terminals generally connect to and extend from respective electrodes for allowing external access to the electrodes. The housing is sealed to preventingress of contaminants and egress of electrolyte.
The energy storage capacity for a supercapacitor can be described by the equation
  E  =            1      2        ⁢          CV      2      where E is the energy in joules, C is the capacitance in farads and V is the rated or operating voltage of the supercapacitor. The distinguishing feature of supercapacitors are the particularly high values of capacitance. Another measure of supercapacitor performance is the ability to store and release the energy rapidly; this is the power, P, of a supercapacitor and is given by
  P  =            V      2              4      ⁢      R      where R is the internal resistance of the supercapacitor. For electrochemical capacitors and batteries, it is more common to refer to the internal resistance as the equivalent series resistance or ESR. As can be deduced from the foregoing equations, the power performance is strongly influenced by the ESR of the entire device, and this is the sum of the resistance of all the materials, for instance, carbon, substrate, binder, separator, electrolyte and the contact resistances as well as the external contacts.
The product of resistance and capacitance (RC), commonly referred to as the time constant, is frequently used to characterise capacitors. In an ideal capacitor, the time constant is frequency independent. However, in carbon based supercapacitors, both R and C are frequency dependent. This arises from the micro-porous characteristics of high surface area carbons, and the nature of charge build up at the electric double layer on the carbon surface. The traditional method of measuring R and C for supercapacitors is to use a constant current charge or discharge and to measure the voltage jump at the start or finish of the cycle, and the rate of change of voltage during the cycle respectively. This however, effectively provides the R at high frequency and the C at low frequency. Another more suitable method is to measure the frequency response of the complex impedance and to model a simple RC element to the data. This provides an estimate of R and C across the frequency range that may or may not correlate with those measured using constant current techniques. Clearly, the use of RC time constant as a measure of capacitor suitability is subject to a large uncertainty. A more useful technique has recently been proposed in which R and C are measured at the frequency at which the phase angle of current and voltage is −45°. The reciprocal of this frequency is the “response time” and is more clearly defined than other methods. Further, the capacitance at this frequency can then be used to calculate the energy and provide a Figure of Merit (FOM) when normalised with respect to the mass or volume of the supercapacitor.
It will be appreciated that a gravimetric FOM is a figure of merit more appropriate for use with energy storage devices intended for pulse power applications. That is, such applications are by necessity frequency dependent and, as such, the calculation of the figure of merit involves first identifying the frequency fo at which the impedance of the storage device reaches a −45° phase angle. A reciprocal of fo then provides a characteristic response time To for the storage device. The value of the imaginary part of the impedance Z″ at fo is used to calculate the energy Eo that the device is able to provide at that frequency. More particularly:
      E    o    =            1      2        ⁢          CV      2      where C=−1/(2πfoZ″) and V is the rated voltage of the device. The gravimetric figure of merit is then calculated by dividing Eo by the mass of the device and by To. That is, gravimetric FOM=Eo/(m·To).
The gravimetric figure of merit has been suggested by John R. Miller in a paper entitled “Pulse Power Performance of Electrochemical Capacitors: Technical Status of Present Commercial Devices” for the “8th International Seminar on Double Layer Capacitors and Similar Energy Storage Devices”, Deerfield Beach, Fla., Dec. 7-9, 1998. The teachings of and disclosure within that paper are incorporated herein by way of cross-reference.
Also detailed in the Miller paper is the calculation of a volumetric figure of merit (volumetric FOM) which is based upon Eo divided by both To and the volume of the device. The volumetric FOM is expressed in terms of Watts/cm3.
These figures of merit provide a different characterisation of storage devices which is more in keeping with the frequency dependent nature of pulse power and other such applications to which the devices are being applied. It should also be noted that the performance of the devices can not be adequately explained by the hitherto utilised simple RC model. Such simple models do not account for the frequency dependent nature of either pulsed or high power applications, whereas the FOM used to characterise the present invention is a parameter directly relevant to such applications.
Another figure useful in assessing the performance of a supercapacitor is Effective Capacitance (Ce). Effective Capacitance (Ce) is the capacitance obtained during a constant current discharge at a specified time and is derived from an RC electrical model of the supercapacitor's measured discharge, where R (or ESR) is measured at a predetermined time, say 20 μs (microseconds) and held constant in the model. The discharge current used here is typically 100 mA. Ce is thus time dependant. The weight used here to calculate the specific gravimetric Effective Capacitance in a supercapacitor is generally the total mass of the device. For dissimilarly packaged or structured devices, a comparison of Ce may be made by comparing the mass of the active coatings, or active materials within coatings, for the devices.
Each electrode may be formed from a single sheet which may be folded or rolled and multiple flat sheets stacked together and electrically connected in parallel. This is referred to here as an electrode stack, such as is described in more detail in commonly-owned publication WO/2000/034964.
There is a relationship between the footprint of an electrode stack and the thickness required to meet predetermined ESR and capacitance values. ESR varies primarily with electrode area, so a smaller footprint requires a proportionally larger number of layers to maintain the same ESR. Capacitance varies with volume of coating, so smaller footprints can be at least partially compensated for by thicker coatings.
In many cases, the physical and electrochemical properties of the electrolyte are a key factor in determining the internal resistance (ESR) of the supercapacitor and the “frequency response” of the supercapacitor, i.e. the ability of the supercapacitor to provide power over various frequency ranges.
The factors influencing the conductance (κ) of an electrolyte solution are described in detail in an article by B. E. Conway taken from “The Fourth International Seminar on Double Layer Capacitors and Similar Energy Storage Devices”, Dec. 12-14, 1994, held at Ocean Resort Hotel and Conference Centre, Deerfield Beach, Fla. and co-ordinated by Florida Educational Seminars, Inc., 1900 Glades Road, Suite 358, Boca Raton, Fla. 33431.
In summary, there are two principle factors which are involved in determining the conductance—these are:
a) the concentration of free charge carriers, cations and anions; and
b) the ionic mobility or conductance contribution per dissociated ion in the electrolyte.
There are a number of sub factors which in turn influence these two principle factors. These are:                The solubility of the selected salt.        The degree of dissociation into free ions and factors such as the extent of ion-pairing of the ionic species. This in turn is influenced by the salt concentration, temperature and the dielectric constant of the solvent.        The viscosity of the solvent, which is a temperature dependent property. As temperature increases, there is a corresponding decrease in viscosity.        
Solvents for supercapacitors can thus be designed with the following criteria in mind:                Solvency for selected ionic species        Degree of dissociation of cation/anion pairing in solution        Dielectric constant        Electron-pair donicity        Permits high ion mobility        Extent of solvation of free ions and radii of solvated ions        Temperature coefficient of viscosity (i.e. low viscosity in the intended temperature range); and        Ion pairing equilibria.        
There is also the necessity for the electrolyte to be chemically stable. Aqueous electrolytes, such as sulphuric acid and potassium hydroxide solutions, are often used as they enable production of an electrolyte with high conductivity. However, water is susceptible to electrolysis to hydrogen and oxygen and as such has a relatively small electrochemical window of operation outside of which the applied voltage will degrade the solvent. In order to maintain electrochemical stability in applications requiring a voltage in excess of 1.0 V, it is necessary to employ supercapacitor cells in series, which leads to an increase in size, a reduction in capacitance and an increase in ESR in comparison to a non-aqueous device which is capable of producing an equivalent voltage. Stability is important when one considers that the supercapacitors may remain charged for long periods and must charge and discharge many hundreds of thousands of times during the operational lifetime of the supercapacitor.
There are of course processing requirements on the solvent also, such as cost, toxicity, purity and dryness considerations for non-aqueous systems.
Non aqueous solvents commonly used in related fields, e.g. batteries, can be classified as: high dielectric constant aprotic (e.g. organic carbonates), low dielectric constant with high donor number (e.g. dimethoxyethane, tetrahydrofuran or dioxolane), low dielectric constant with high polarisability (e.g. toluene or mesitylene) or intermediate dielectric constant aprotic (e.g. dimethylformamide, butyrolactone) solvents.
However, in addition to the specific electrolyte requirements of supercapacitors mentioned above, there is also the practical consideration that supercapacitors do not operate in isolation. Rather, in use, they are in confined environments in the presence of components which generate high temperatures, and like the other components, this must be borne in mind when selecting the electrolyte and/or electrolyte solvent. Also, it needs to be borne in mind that the supercapacitors must be capable of operation at start-up at temperatures much lower (even into the sub zero range) than the high operating temperatures referred to above.
The use of supercapacitors in environments that require dimensionally stable housings has become highly desirable. Supercapacitors in soft packages are suitable for use in small numbers of high-end hand held devices and the like, however, soft packages are not so suitable for use in demanding environments, such as motor vehicles or where highly automated assembly is used. The use of rigid packages, which can be surface mounted is desirable, however, surface mount technology (SMT) has its own drawbacks, including instances of thermal shock during manufacture.
The soldering of a supercapacitor to a circuit board in a surface mount process, such as reflow, creates high temperatures over a relatively short time period. This heat is in part conducted along a supercapacitor's terminal and into the body of the supercapacitor and in part through the package walls. Due to its generally small mass, a supercapacitor is therefore susceptible to large thermal shock, as only a small amount of heat is required to raise its temperature rapidly. Where large thermal shock is experienced, expansion, separation or vaporisation of the electrolyte may occur, which may cause performance impairment and may ultimately lead to the supercapacitor's destruction. Considerable care must therefore be taken in selecting an electrolyte and/or electrolyte solvent, which is capable of withstanding short bursts of high thermal energy.